I'm a bit late to this thread, but I have some experience with the topic.
I bought my first M77 in 1973 or 74 in 308 Winchester. As a young buck sergeant, it was a pretty significant hit on the pay check, even with jump pay. It was a standard weight rifle. With M118 White Box match ammo, it wouldn't hold 4 MOA even after playing with the bedding, both the action and the barrel. It was quickly sold.
The second one was a heavy barrel in 308 in 1976 as I recall. It would seize up with factory ammo and required a wood mallet to open the bolt after almost every shot, even with a cold barrel. It was DXd pretty fast.
In the 70s, metallic silhouette was pretty popular and standard rifles were still competitive so I, like any psychotic, bought a third heavy barrel M77 somehow expecting a different result. The third rifle was pretty accurate with the White Box match ammo, but either the barrel was screwed in crooked or the sight bases were cast out of alignment as any scope tried on it didn't have enough right windage to zero up at a 100 yards. With windage dialed in all the way to the right with a top of the line Redfield scope, it was still about 8 inches left of the point of aim. It too went the way of the first two M77s.
I have't bought one since.
My Dad just gave me his M77 Mk II in .223. A quick trip to the range last week showed it key holing badly at 100 yards with my mid range match load of 69 grain Sierra Match Kings. Suspecting that it has a 1 in 12 twist, I tried some 55 grain FMJ bullets through it for satisfactory results of about 1 1/2 MOA through 200 yards. Now I just need to replace or fix the absolutely horrible trigger on it before using it on prairie dogs or coyotes. As it was a gift, I'll probably keep it... I guess.
I'm glad to hear that others have had more positive experiences with the M77. I've always felt that it was a good looking rifle and my nephew slays deer and elk every year with his 30/06 M77.